Add clear indication for path liveness and permissions changes

[gavinleroy]

Aquascope shows how permissions change to help users understand why a particular permission may be missing. Per path, these changes are described as a gain or loss in permissions. Additionally, we use a single icon to indicate why these changes occurred, I'll refer to these icons as the change reason.

In the above example, this table row tells the reader that the path s gained RWO permissions and the up arrow indicates that this was because the variable became live. However, showing a single icon has proven to be insufficient because, with liveness, multiple change reasons can exist. Consider line 3 in the below example:

On line 3, the path s loses RWO permissions. A common question from readers is "hey, shouldn't s keep R permissions because the path is only borrowed immutably?" The answer is YES. A path borrowed immutably can still be read. The issue is that s is no longer used after line 3, it dies; however, because our visual indication only allows for a single icon this change in liveness is not called out to the reader.

The task here is to develop a system of indication that is flexible enough to convey multiple change reasons. The result should achieve the following goals: the visualization should remain clear, and cases where only one change reason exists should not become more confusing. In the past, we've experimented with showing multiple icons, and a dropdown list of icons, but both of these were insufficient as they violated the aforementioned goals.

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The previous code for easy copy/pasting.

fn foo() {
  let mut s = String::from("");
  let b = &s;

  println!("{b}");
}

[willcrichton]

For example, one possibility could be to annotate individual permissions with their cause when multiple causes exist, like in this example:

R↓ W→ O→